The accidental discoveries in alloying have transformed the standard of living for the civilizations, across centuries. A careful review in development of alloying suggests that there have been significant progress in the field of metallic alloys in the 20th century. These developments open doors of new discoveries and opportunities. It is evident from the fact that the materials engineers have attempted to think beyond conventional binary and ternary alloys. The multi-component alloying has been attempted, especially in case of metallic glasses and high entropy alloys. This book is an attempt to review the landmark development in the field of compositional design of metallic glasses and high entropy alloys. Further, an attempt has been made to identify the challenge in the course of development of the subject and future directions.